[sara-mokhtari]

Description

Fixes #775
This model describes a core-shell particle with a bifrustum core and a prism shell. Both shapes are defined with their symmetry axis along the z-axis and centered at the origin. The bifrustum core can be rotated around the z-axis by an angle alpha (core_rotation) and translated along the z-axis by a distance z_0 (core_translation) with respect to the prism shell.

Depends on #781 (support for pure Python 2D models): this PR is based on py-Iqabc and targets it,
so the diff only shows the bifrustum. Once #781 is merged, I will retarget this PR to master
and merge master into this branch.

• F(q) amplitude is given by the sum of the prism and bifrustum form factors,
  weighted by their respective scattering length densities (sld_shell, sld_core)
  and the solvent scattering length density (sld_solvent).
• 1D: orientation average with a Fibonacci quadrature (500 points)
• 2D: Fqabc/Iqabc with theta, phi, psi

Note for review: the model structure is inspired from mixture.py, with parameters having a prefix "core_" or "shell_" (ex: core_radius, shell_radius). The idea is to start generalizing the model. Also, prism.py was updated to be able to call directly Fqabc without flattening qa, qb, qc beforehand (important if we want to make a general core-shell model at the end).

How Has This Been Tested?

• model_test passes
• F(q) checked against an independent method (Debye equation with Debye calculator: https://github.com/FrederikLizakJohansen/DebyeCalculator) for different number of sides of the prism. The Debye equation is based on atomic positions while SasView model is based on analytical expressions. Not tested on experimental data yet.

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

[sara-mokhtari]

2D orientation question:

• In the other models (ex: tetrahedron): _Iqabc uses Fqabc(qa,qb,qc, *args) without the sld and the scale factor and Iqabc is the one that uses the sld and the scale factor.

• In the core-model, the sld are directly taken into account in the computation of Fqabc(qa, qb, qc, sld_core, sld_shell, sld_solvent, *args). So in this case, _Iqabc uses the sld. Here is how I defined the two functions:

def _Iqabc(qa, qb, qc, sld_core, sld_shell, sld_solvent, *args):
    A = Fqabc(qa, qb, qc, sld_core, sld_shell, sld_solvent, *args)  # shape (nq, npoints)
    intensity = (np.abs(A))**2  # (1e-6/Ang^2)**2 = 1e-12/Ang^4
    return intensity

def Iqabc(qa, qb, qc, sld_core, sld_shell, sld_solvent, *args):
    intensity = _Iqabc(qa, qb, qc, sld_core, sld_shell, sld_solvent, *args)
    return intensity * 10**-4

Since I do not really understand how the functions must be defined for 2D orientations (how the API works), I think this part should be carefully reviewed.

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